Leaf Defier guards have biocide and fungicide built in to prevent organic growth in the gutters.
Baking soda, for example, makes an effective fungicide.
Use a fungicide as directed to counteract this problem.
Another options is having an arborist spray a special fungicide containing a chemical called mancozeb on the trees.
It is an insecticide, disinfectant, germicide, fungicide, deodorant, bactericide and bacteriostatic all-in-one.
Care must be taken in using this fungicide not to wet the painted wood, as it is sure to become discoloured.